Well, each parent knows his/her kids best of course, but I can't fathom even having doubts as to whether kids over age 10 or so would be okay alone at the parks. I have a different perspective on it, since I grew up in the Orlando area. I went to day camp every summer, and we regularly did field trips to the theme parks. Universal didn't open till I was 14, but we used to go to Disney, SeaWorld, Busch Gardens, even the water parks.
The day camp rule around here is simple, and standard among several different programs: Under age 7 must stick with a counselor. Over age 7 must use the buddy system -- no going off by yourself, but two or more kids together are fine. So I've been in the parks without adult supervision since the age of 7, and I lived.
I graduated high school a bit early, and when I was 16 I had my own apartment and a job at Universal. I guess I grew up fast, but it certainly didn't hurt me...nor did it make me stop wanting to tour the parks with my parents -- back in the day, Universal team members got VIP back door access to all attractions, and I used to take my parents to the park every weekend. We could hit every attraction multiple times before my shift started at 3.
I know it's hard to let your kids grow up, but they're generally a lot more capable than we give them credit for. Just my two cents of course.